Rotor losses in permanent magnet electric machines are caused by power electronics switching harmonics imposed on the stator windings, and in turn, on the electrically conductive materials of the rotor. These very high-frequency harmonics cause eddy currents in the rotor, and thus I2R (current squared×resistance) losses. Reducing rotor losses in high-speed permanent magnet motors becomes more challenging as the rotating speed and switching frequency increase.
Removing rotor losses has always been an issue, as the only effective methods are conduction to the bearings and mass of the rest of the rotor and convection to airflow through the machine air gap. Neither are good at removing a significant amount of loss from the rotor. Segmentation of the rotor magnets can help to reduce rotor losses at relatively low switching frequencies. However, such segmentation is impractical for high switching frequency, high-speed machines, as the degree of segmentation necessary to gain appreciable improvements in losses detracts substantially from the magnet fill factor of the rotor.
Like reference symbols in the various drawings indicate like elements.